1. Field of the Invention
The present invention relates to a micromirror unit used in optical apparatus for the purposes of changing the direction of light. In particular, it relates to a micromirror unit of the type which is advantageously incorporated in an optical disk apparatus (for writing to or reading data from an optical disk), an optical switching apparatus (for selectively connecting one optical fiber to another to provide a light passage), etc.
2. Description of the Related Art
A micromirror unit is provided with a reflective mirror member which is pivotable for changing the direction of reflected light. A popular technique for actuating the mirror member is to utilize electrostatic force. Micromirror units of this type (referred to as “static driving type” hereinafter) may have several structures. Such micromirror units are generally classified into two groups, depending on fabrication methods. One of the methods employs a “surface micro-machining” technique, whereas the other employs a “bulk micro-machining” technique.
In accordance with the surface micro-machining, patterned material layers in lamination may be formed on a base substrate, thereby providing required components such as a support, a mirror member and electrodes. In this layer forming process, a dummy layer (or sacrificial layer), which will be removed later, may also be formed on the substrate. A conventional micromirror unit of the static driving type by the surface micro-machining is disclosed in JP-A-7 (1995)-287177 for example.
In accordance with the bulk micro-machining, on the other hand, a base substrate itself is subjected to etching, thereby providing required components such as a frame and a mirror forming base. Then, a mirror member and electrodes may be formed on the etched substrate by a thin-film forming technique. Micromirror units of the static driving type by the bulk micro-machining are disclosed in JP-A-9 (1997)-146032, JP-A-9-146034, JP-A-10 (1998)-62709 and JP-A-2000-13443.
One of the technically significant factors desired in a micromirror unit is a high flatness of the reflective mirror member. According to the above-mentioned surface micro-machining technique, however, the thickness of the resulting mirror member is rendered very small, so that the mirror member is liable to warp. To avoid this and ensure a high flatness, the mirror member should be made so small that its respective edges are less than 100 .mu.m in length. In accordance with the bulk micro-machining, on the other hand, a rather thick substrate is processed, thereby providing a sufficiently rigid mirror forming base to support the mirror member. Thus, a relatively large mirror member having a high flatness can be obtained. Due to this advantage, the bulk micro-machining technique is widely used to fabricate a micromirror unit having a large mirror member whose edges are more than 100 .mu.m in length.
FIG. 10 of the accompanying drawings shows an example of conventional micromirror unit fabricated by the bulk micro-machining technique. The illustrated micromirror unit 400 is of the static driving type, and includes a lamination of a mirror substrate 410 and a base substrate 420. As shown in FIG. 11, the mirror substrate 410 includes a mirror forming base 411 and a frame 413. The mirror forming base 411 has an obverse surface upon which a mirror member 411a is formed. The mirror forming base 411 is supported by the frame 413 via a pair of torsion bars 412. The mirror forming base 411 has an reverse surface upon which a pair of electrodes 414a and 414b is formed. As shown in FIG. 10, the base substrate 420 is provided with a pair of electrodes 421a and 421b which faces the above-mentioned pair of electrodes 414a and 414b of the mirror forming base 411.
With the above arrangement, the electrodes 414a, 414b of the mirror forming base 411 may be positively charged, whereas the electrode 421a of the base substrate 420 may be negatively charged. As a result, an electrostatic force is generated between these electrodes, thereby turning the mirror forming base 411 in the N3-direction shown in FIG. 10 as the torsion bars 412 are being twisted. The rotation angle of the mirror forming base 411 is determined by the balance between the inter-electrode electrostatic force and the restoring force of the twisted torsion bars 412. To rotate the mirror forming base 411 in the opposite direction, the other electrode 421b of the substrate 420 may be negatively charged. As readily understood, when the mirror forming base 411 is turned clockwise or counterclockwise, as required, the light reflected on the mirror member 411a is directed in the desired direction.
As noted above, the mirror forming base 411 is rotated through an angle which is defined by the balance between the inter-electrode electrostatic force and the restoring force of the twisted torsion bars 412. Thus, it is possible to adjust the rotation angle of the base 411 by controlling the static electricity to be generated in correlation with the restoring force of the torsion bars 412.
Generally, a micromirror unit is a structure whose minimum dimension is about several hundred micrometers. This is rather large size, and therefore the restoring force of the torsion bars tends to exceed the inter-electrode electrostatic force in strength. Thus, conventionally, the area of each electrode is rendered large (for generating a great electrostatic force), whereas each torsion bar is made uniformly thin along its length (for weakening the restoring force). In the prior art micromirror unit 410 (FIG. 11), each torsion bar 412 has a constant small width L along the entire length.
In the above manner, however, the mirror forming base 411 is supported by the thin torsion bars 412. Accordingly, it is difficult to hold the mirror forming base 411 stable (i.e., nonrotatable) about the normal N3 (the line at right angles to the surface). If unstable about the normal N3, the mirror forming base 411 is liable to unduly swivel about the normal N3 when the base 411 is supposed to rotate only about the axis defined by the torsion bars 412. When such an unwanted swivel occurs, it is difficult or even impossible to precisely control the operation of the micromirror unit.